The present invention relates to picture image forming equipment, such as a copying machine, a facsimile machine, or a printer, of the type generally used in an electrostatic recording process, which consists of forming an electrostatic latent image on an image carrier, such as a photosensitive material and then obtaining or "developing" a picture image by converting such an electrostatic latent image into a visible image. The present invention relates more particularly to improvements of a fixing unit, which fixes a yet unfixed, developed picture image by applying a heating process thereto.
FIG. 3 illustrates a perspective view of a conventional fixing unit 1.
This fixing unit 1 is comprised of a heating roller 2, which has a heating source such as a lamp light source (not shown in FIG. 3) set therein, and a pressure roller 3, which contacts the heating roller 2 under pressure. In a photosensitive material and developing unit, not shown in FIG. 3, a toner image (a picture image) not yet fixed is formed on paper, and the paper is subsequently transported to the fixing unit 1, which fixes the toner image on the paper by heat from the heating roller 2 while it transports the paper between the heating roller 2 and the pressure roller 3.
Moreover, in FIG. 3, the reference number 8, indicates a cleaner made of felt which wipes off excess toner sticking to the circumferential area of the heating roller 2. The reference number 4 indicates a plate spring, which applies pressure to the cleaner 8 to keep it in contact with the circumferential surface of the heating roller 2. Reference number 5 indicates a heat cover, which is designed to secure the thermal insulation of the fixing unit 1.
In still further detail, the conventional fixing unit 1 described above is provided with the pressure roller 3 in a unified structure made of silicone rubber 6, as illustrated in the enlarged cross-sectional view of the relevant parts in FIG. 4. Consequently, the pressure roller 3 has a comparatively high degree of hardness as a whole.
The conventional fixing unit 1 described above, having pressure roller 3 is made of a unified structure of silicone rubber 6, because it undergoes a rapid aging change and consequently has a short service life.
Also, the pressure roller 3 thus made in a substantially monolithic structure of silicone rubber 6 is subject to a deformation of its surface. During this deformation, the part of the surface of the pressure roller 3 which is in contact with the paper has such a considerable pressure variation in the width direction of the paper while the fixing unit transports the paper between the heating roller 2 and the pressure roller 3 that a difference in the paper transporting speed causes deformation of the surface of the pressure roller 3 and wrinkles in the paper.
Moreover, since the pressure roller 3 is formed of silicone rubber 6, the pressure roller has the roller likely to collect the toner on its surface and to be smeared with such toner.
Furthermore, since the conventional fixing unit 1 is provided with a pressure roller 3 which is formed of a unified structure of silicone rubber 6 which has a high degree of hardness as a whole, the pressure roller 3 can hardly achieve adequate contact with the heating roller 2. As a result, the conventional fixing unit 1 that it cannot produce fixed images of high picture quality from the yet unfixed toner images formed on the paper by bringing such images into a sufficiently close and uniform contact with the heating roller 2.